Mechanisms which control the frequency distribution of polyribosome-associated messenger RNA sequences are being studied in a parent and chemically-transformed derivative line of AKR mouse embryo cells in culture. Molecular cloning techniques are being utilized to derived a number of individual cloned sequences corresponding to mRNAs present in differing intracellular frequency in the two cell types. These mRNAs include transcripts of endogeneous AKR-murine leukemia virus (MuLV)-related genes, alpha and beta globin genes, and randomly selected genes coding for unknown products. These cloned sequences will then be utilized in molecular hybridization experiments to measure the relative nuclease sensitivity of the corresponding genes in isolated chromatin, and the relative rates of synthesis, processing, and cytoplasmic metabolism of each corresponding mRNA sequence. The results of these experiments whould allow an evaluation of the relative contribution of transcriptional and post-transcriptional mechanisms in the regulation of gene expression and may provide insight into the lesions in these mechanisms which accompany chemical transformation of cells in culture.